blogs__class-decorator

JavaScript class decorator

Using class decorator to automatically attach instance to the window

The application source code uses a class decorator to automatically save any created instances on the window object when running inside Cypress.

// src/components/TodoList.jsx
import { CypressInstances } from '../decorators'

@CypressInstances()
export default class TodoItem extends Component {
  ...
}

During the test, we can get to that component using cy.window().its('TodoItem') to check internal state or trigger component's methods.

// cypress/e2e/spec.cy.js
it('creates Todo components', () => {
  cy.visit('/')
  cy.get('.new-todo')
    .type('first{enter}').type('second{enter}').type('third{enter}')

  cy.window().its('TodoItem').should('have.length', 3)
    // let's complete second todo
    .its('1.props').then((props) => {
      props.completeTodo(props.todo.id)
    })

  // UI updates
  cy.get('li.todo').eq(1).should('have.class', 'completed')
})

Note: class decorators are an experimental JavaScript feature. In this recipe they are transpiled using Babel plugins in the .babelrc file.

There are two class decorators in the src/decorators/index.js file.

import {CypressSingleton, CypressInstances} from './decorators'

// use CypressSingleton if there is only one instance of a class
// for example Todo application only has a single input component
@CypressSingleton('input')
class TodoFormInput {
  ...
}
// from the test use
cy.window().its('input') // yields TodoFormInput

// there might be multiple Todo item components
@CypressInstances('todos')
class TodoItem {
  ...
}
// from the test use
cy.window().its('todos') // yields TodoItem[]